A construction of a conventional plane motor is described with reference to FIG. 10 disclosed in Japanese Patent Application Laid-Open No. 2002-112256.
Japanese Patent Application Laid-Open No. 2002-112526 discloses a plane motor 20 capable of driving in the direction of two axes, X and Y. The plane motor 20 comprises a movable stage (movable element) 22 having a plurality of magnets 38, and a stator 26 having a group of coils consisting of a plurality of coils 54 corresponding to driving in the X-axis direction and a group of coils consisting of a plurality of coils 56 corresponding to driving in the Y-axis direction. Applying power to appropriate coils 54 (or 56) arranged opposite to the plurality of magnets 38 enables generation of a Lorentz force in the X-axis (or Y-axis) direction on the movable stage 22. By controlling the amount of power application, the movable stage 22 can be driven in two-dimensional directions. Note that two layers of coils 54 are provided in the Z-axis direction as coils corresponding to driving in the X-axis direction, and two layers of coils 56 are provided in the Z-axis direction as coils corresponding to driving in the Y-axis direction.
Shown in FIGS. 5A and 5B are a top view and a sectional side view of a plane motor having another construction. Shown in FIG. 6 is an enlarged view of the part A shown in FIG. 5B.
The plane motor shown in FIGS. 5A, 5B and 6 comprises a stator 10, and a movable element 20 having a magnet array 21 consisting of a plurality of magnets fixed under the plane stage top board 22. The stator 10 is configured with four layers of coil arrays 1 to 4. From the top layer, the first-layer coil array 1 is provided for driving in the X-axis direction or ωx-axis direction. The respective coil arrays are arranged in the space (refrigerant channel F) between a base 6 and a partition 7. In the space between the base 6 and the partition 7, refrigerant is introduced from a refrigerant inlet 8 in the upstream portion and emitted from a refrigerant outlet 9 in the downstream portion.
Herein, the respective layers of coil arrays are spaced at constant intervals in view of the refrigeration efficiency. Furthermore, as shown in FIGS. 7A and 7B, since the magnet array 21 of the movable element 20 is asymmetrical, even when all the coil arrays are provided as long coils, it is possible to generate a moment of force in the movable element by appropriately selecting the coil arrays to be energized. Also, by appropriately applying power to respective coil arrays, the movable element can be controlled in the rotational direction. Note that “long” in the long coils means that the effective length of the coils is larger than the magnet array 21 arranged in the movable element as shown in FIGS. 5A and 5B, and “short” means that the effective length of the coils is smaller (shorter) than the magnet array arranged in the movable element 22, as with the first-layer coil array shown in FIG. 10.
In general, the larger the space (magnetic gap) between the magnets 38 of the movable element 22 and the coil array 54 (56) of the stator, the smaller the magnetic flux density. Therefore, in the construction shown in FIG. 10, the magnetic flux density in the neighborhood of the coil array 56 provided for driving in the Y-axis direction, which has a relatively larger magnetic gap, is smaller than the magnetic flux density in the neighborhood of the coil array 54 provided for driving the X-axis direction. For this reason, the Lorentz force (i.e., driving force constant) obtained in a case in which the same amount of current is applied to the respective coil arrays is smaller in the Y-axis driving than the X-axis driving. Since an ordinary positioning apparatus requires an equal driving force in the X-axis direction and the Y-axis direction, the heat generation amount of coil array 56 for Y-axis driving is larger than that of the coil array 54 for X-axis driving.
FIG. 8 is a graph showing a trend of a driving force constant corresponding to the magnetic gap of coils, assuming a case wherein each coil array has an equal conductive area of a section having a normal in the coil winding direction. As shown in the graph, the larger the distance between the coils and the magnets (i.e., the larger the magnetic gap of the coils), the smaller the driving force constant of the coils. This trend become more significant as the magnetic gap becomes smaller. In other words, although two layers of coil arrays are provided in FIG. 10, it is supposed that the driving force constant of the coil array 56 for Y-axis driving, which is arranged in the lower layer having a large magnetic gap, is considerably smaller than the driving force constant of the coil array 54 for X-axis driving. Furthermore, when an equal driving force is to be generated, since the heat generation amount of coils is inversely proportional to a square of the driving force constant, it is supposed that the heat generation amount of the coil array 56 for Y-axis driving becomes extremely larger compared to that of the coil array 54 for X-axis driving. From the aspect of coil refrigeration, heat of coils that are positioned with a large magnetic gap often becomes a problem.
Furthermore, when coils are provided in multiple layers, as shown in FIG. 6, the difference in the driving force constant is even larger between the highest-layer coil array 1 and the lowest-layer coil array 4. In a case wherein inert refrigerant is provided from the refrigerant inlet 8 to the outlet 9 to directly cool down the entire coil arrays, as shown in FIGS. 5A, 5B and 6, it is necessary to determine the amount and temperature of the refrigerant in accordance with the lowest-layer coil array 4, which has the largest heat generation amount.
The above-description is summarized. Since the plane motor utilizing a Lorentz force has multiple layers of coils in respective driving-axis directions, the magnetic gaps between the magnets and the coil array are different for each driving axis, and the driving force constants of respective driving axes largely differ. In other words, the larger the magnetic gap of the coil array driven in a driving axis, the worse the driving efficiency. As a result, heat generation becomes unbalanced, i.e., while the upper-layer coil array (small magnetic gap) does not cause much heat generation, the lower-layer coil array (large magnetic gap) causes extremely large heat generation. Moreover, in a case of refrigerating the entire coil arrays by refrigerant, because the amount and temperature of the refrigerant are determined in accordance with the coil array causing the largest heat generation amount, a large amount of refrigerant is wastefully supplied to the upper-layer coil array, and an extremely large amount of refrigerant becomes necessary for the stator as a whole.
Therefore, in the plane motor utilizing a Lorentz force, eliminating the unbalanced heat generation in the respective layers of coil arrays, as well as improvement of driving efficiency, are desired from the aspect of efficient coil generation.